UBU

Examples:
:ubu :x-3 ; undo the last three commands (same as :ubt :x-2)
:ubu (:x -3) ; same as above
:ubu fn ; undo back up to, but not including the introduction of fn
; (so fn will continue to be defined)
:ubu 5 ; undo back up to, but not including, the fifth command
; executed (leaving the first five commands in place)